1. Field of the Invention
The present invention relates generally to methods and apparatus for making optical fibers with plurality of holes or stress rods that utilize soot pressing for optical fiber overcladding, and particularly to methods and apparatus for making optical fiber preforms.
2. Technical Background
Conventional chemical vapor deposition (CVD) processes, such as outside vapor deposition (OVD) and vapor axial deposition (VAD) processes, for making optical fiber preforms often utilize only a portion of the starting raw material due to limitations in the deposition efficiency of the OVD process. Use of the resulting “waste” silica soot could, therefore, potentially result in significant raw material cost savings.
Accordingly, different methods have been devised to utilize otherwise unutilized silica soot in the production of optical fiber preforms. These methods can suffer from a variety of drawbacks including expensive, complicated, and/or time consuming processing conditions and equipment, and may result in preforms with less than desirable properties such as unacceptable variability with respect to preform density and geometry.
There are a number of optical fiber applications where plurality of holes or stress rods are used in the cladding to achieve desirable optical properties. The applications include single polarization fiber, polarization maintaining fibers, bend insensitive fibers, photonic crystal fibers, high numerical aperture fibers, and endless single mode fibers.
Single polarization and polarization maintaining fibers typically include a central core and multiple air hole or boron doped stress rods (FIG. 1) situated within a cladding and near the fiber core. These fibers are often manufactured by an outside vapor deposition (OVD) process in which silica cladding glass is deposited on a glass core cane, for example, through the pyrolysis of octamethyltetrasiloxane. The OVD process is a highly optimized, high yield manufacturing process. However, the formation of the cladding layer is often the rate limiting step in maximizing optical fiber output. Further, it is estimated that as little as 50% of the pyrolysis product of the octamethyltetrasiloxane feedstock is deposited on the glass core canes during deposition of the cladding portion of the optical fiber preform. The silica cladding soot layer is then sintered to create a silica core/cladding glass blank. The single polarization and polarization maintaining fibers are usually manufactured by drilling/machining precise holes inside these core/cladding glass blanks and, and if stress rods are utilized, by inserting the stress rods into these holes. The resultant assembly may be inserted into a silica tube or overcladded, sintered, and then drawn into single polarization or polarization maintaining optical fibers. However, in order to obtain good optical performance, the dimensions of the drilled holes must be very precise and require a significant amount of post-processing, resulting in an increase in the processing cost of these fibers.
In another set of applications, a plurality of holes are present in the cladding. Such configurations are suitable for photonic crystal fibers or bend insensitive fiber application. These fibers are generally made using a stack and draw process or by drilling holes in the preform.
In order to further improve optical fiber output and reduce raw material costs and other manufacturing costs, alternative methods of manufacturing optical fibers with plurality of holes or stress rods in the cladding are desirable.